Mice were immunized with membrane enriched fractions of metastatic human mammary carcinoma lesions. Splenic lymphocytes were fused with non-Ig secretor myeloma cells to generate subsequent hybridoma cultures synthesizing thirteen monoclonal antibodies reactive with human mammary carcinoma cells; two of the monoclonals were shown to react with carcinoembryonic antigen. The monoclonals could be placed into six major groups based on (a) reactivity in solid phase RIAs with extracts of metastatic lesions of mammary carcinomas, and (b) binding to the surface of live mammary tumor cells in culture using live cell RIAs and fluorescent activated cell sorter analyses. Some of the monoclonals demonstrated a "pancarcinoma" activity, reacting with the surface of some non-breast carcinomas, but none of the eleven monoclonals reacted with the cell surface of melanomas, sarcomas, various hematopoietic malignancies, and numerous apparently normal cell 1nes. The immunoperoxidase technique was used on fixed tissue sections to determine the extent of reactivity of the different monoclonals with various types of primary mammary tumors and with metastatic lesions in lymph nodes and at distal sites. These antibodies react with approximately 85 percent of human mammary and colon carcinomas.